Amplifiers are frequently used to isolate an input source from a load and generate an output voltage that accurately tracks the input voltage. Amplifiers typically have an input stage and an output stage, where the output stage drives the load. The output stage typically provides a greater current than the input source.
In one example of an amplifier, it is assumed that the input voltage Vin can swing from ground to 5 volts (the Vcc rail voltage) and that the output stage uses a controllable pull-up current source in series with a controllable pull-down MOSFET to drive the load. In a simplified output stage, used to illustrate a problem with achieving a ground output voltage Vout, the source of the MOSFET is connected to ground, the drain is connected to the Vout terminal, and the controllable pull-up current source is connected to the Vout terminal and a 5 volt supply. Ideally, the MOSFET is an open circuit when Vout is intended to be 5 volts and a short circuit when Vout is intended to be at ground. One problem with such conventional output stages is that, when Vin is at ground, the voltage differential across the source-drain (i.e., Vds) of the output MOSFET cannot be at or near zero volts since the MOSFET then operates in its triode mode (operates like a resistor) and this limits the swing to ground. The triode mode is also known as the linear mode or ohmic mode where a slight change in the drain voltage linearly changes the current through the MOSFET (having a constant gate voltage). As the drain voltage (Vout) approaches zero while sinking current, the output voltage cannot swing to ground due to the on-resistance of the MOSFET times the load current. Therefore, the MOSFET cannot fully pull the Vout terminal to ground.
This problem can be overcome by connecting the source of the output MOSFET to a negative voltage (e.g., from a charge pump or external negative power supply) so that there is still a sufficiently high Vds for the MOSFET to pull the Vout terminal to ground (or even below ground), whether the MOSFET operates in its saturation region or its triode region. However, such dual power sources increase power consumption by the amplifier and, in the case of a negative power supply, add cost and board area.
What is needed is an amplifier that can pull its Vout terminal to ground (or below) using a single power supply for its output stage in a power efficient manner.